1. Field of the Invention
The present invention pertains to logging while drilling apparatus and more particularly to acoustic logging while drilling apparatus and attenuation of acoustic pulses that travel parallel to the direction of drilling.
2. Related Prior Art
To obtain hydrocarbons such as oil and gas, wells or wellbores are drilled into the ground through hydrocarbon-bearing subsurface formations. Currently, much current drilling activity involves not only vertical wells but also drilling horizontal wells. In drilling, information from the well itself must be obtained. While seismic data has provided information as to the area to drill and approximate depth of a pay zone, the seismic information can be not totally reliable at great depths. To support the data, information is obtained while drilling through logging while drilling or measuring while drilling (MWD) devices. Logging or measuring while drilling has been a procedure in use for many years. This procedure is preferred by drillers because it can be accomplished without having to stop drilling to log a hole. This is primarily due to the fact that logging an unfinished hole, prior to setting casing if necessary, can lead to washouts, damaging the drilling work that has already been done. This can stall the completion of the well and delay production. Further, this information can be useful while the well is being drilled to make direction changes immediately.
Advances in the MWD measurements and drill bit steering systems placed in the drill string enable drilling of the horizontal boreholes with enhanced efficiency and greater success. Recently, horizontal boreholes, extending several thousand meters (“extended reach” boreholes), have been drilled to access hydrocarbon reserves at reservoir flanks and to develop satellite fields from existing offshore platforms. Even more recently, attempts have been made to drill boreholes corresponding to three-dimensional borehole profiles. Such borehole profiles often include several builds and turns along the drill path. Such three dimensional borehole profiles allow hydrocarbon recovery from multiple formations and allow optimal placement of wellbores in geologically intricate formations.
Hydrocarbon recovery can be maximized by drilling the horizontal and complex wells along optimal locations within the hydrocarbon-producing formations. Crucial to the success of these wells is establishing reliable stratigraphic position control while landing the well into the target formation and properly navigating the drill bit through the formation during drilling. In order to achieve such well profiles, it is important to determine the true location of the drill bit relative to the formation bed boundaries and boundaries between the various fluids, such as the oil, gas and water. Lack of such information can lead to severe “dogleg” paths along the borehole resulting from hole or drill path corrections to find or to reenter the pay zones. Such well profiles usually limit the horizontal reach and the final well length exposed to the reservoir. Optimization of the borehole location within the formation also can have a substantial impact on maximizing production rates and minimizing gas and water coning problems. Steering efficiency and geological positioning are considered in the industry among the greatest limitations of the current drilling systems for drilling horizontal and complex wells. Availability of relatively precise three-dimensional subsurface seismic maps, location of the drilling assembly relative to the bed boundaries of the formation around the drilling assembly can greatly enhance the chances of drilling boreholes for maximum recovery. Prior art down hole devices lack in providing such information during drilling of the boreholes.
Modem directional drilling systems usually employ a drill string having a drill bit at the bottom that is rotated by a drill motor (commonly referred to as the “mud motor”). A plurality of sensors and MWD devices are placed in close proximity to the drill bit to measure certain drilling, borehole and formation evaluation parameters. Such parameters are then utilized to navigate the drill bit along a desired drill path. Typically, sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a formation resistivity measuring device are employed to determine the drill string and borehole-related parameters. The resistivity measurements are used to determine the presence of hydrocarbons against water around and/or a short distance in front of the drill bit. Resistivity measurements are most commonly utilized to navigate the drill bit. However, the depth of investigation of the resistivity devices usually extends only two to three meters and resistivity measurements do not provide bed boundary information relative to the downhole subassembly. Furthermore, the location of the resistivity device is determined by some depth measuring apparatus deployed on the surface which has a margin of error frequently greater than the depth of investigation of the resistivity devices. Thus, it is desirable to have a downhole system which can accurately map the bed boundaries around the downhole subassembly so that the drill string may be steered to obtain optimal borehole trajectories.
The relative position uncertainty of the wellbore being drilled and the critical near-wellbore bed boundary or contact is defined by the accuracy of the MWD directional survey tools and the formation dip uncertainty. MWD tools may be deployed to measure the earth's gravity and magnetic field to determine the inclination and azimuth. Knowledge of the course and position of the wellbore depends entirely on these two angles. Under normal conditions, the inclination measurement accuracy is approximately plus or minus two tenths of a degree. Such an error translates into a target location uncertainty of about three meters per one thousand meters along the borehole. Additionally, dip rate variations of several degrees are common. The optimal placement of the borehole is thus very difficult to obtain based on the currently available MWD measurements, particularly in thin pay zones, dipping formations and complex wellbore designs.
Until recently, logging while drilling has been limited to resistivity logs, gamma logs, neutron logs and other non-acoustic logs since acoustic noise caused by drilling and acoustic pulses traveling upstring from the transmitter has presented problems in accurate detection and delineation. These problems cannot be easily isolated by arrival time since the acoustic pulses are generated and detected continuously. Recently, the use of acoustic sensors having a relatively short spacing between the receivers and the transmitter to determine the formation bed boundaries around the downhole subassembly has been used. An essential element in determining the bed boundaries is the determination of the travel time of the reflection acoustic signals from the bed boundaries or other interface anomalies. A prior art proposal has been to utilize estimates of the acoustic velocities obtained from prior seismic data or offset wells. Such acoustic velocities are not very precise because they are estimates of actual formation acoustic velocities. Also, since the depth measurements can be off by several meters from the true depth of the downhole subassembly, it is highly desirable to utilize actual acoustic formation velocities determined downhole during the drilling operations to locate bed boundaries relative to the drill bit location in the wellbore.
Additionally, for acoustic or sonic sensor measurements, the most significant noise source is acoustic signals traveling from the source to the receivers via the metallic tool housing and those traveling through the mud column surrounding the downhole subassembly (tube waves and body waves). In some applications acoustic sensor designs are used to achieve a certain amount of directivity of signals. A transmitter coupling scheme with signal processing method may be used for reducing the effects of the tube wave and the body waves. Such methods, however, alone do not provide sufficient reduction of the tube and body wave effects, especially due to strong direct coupling of the acoustic signals between the transmitters and their associated receivers.
Some U.S. patents representative of the current art in determining subsurface formations are as follows.
U.S. Pat. No. 4,020,452, titled “Apparatus For Use in Investigating Earth Formations”, issued to Jean-Claude Trouiller, et al., relates to an apparatus for mechanically filtering acoustic pulses in a well logging tool. This apparatus includes of a substantially rigid member having interruptions in the longitudinal continuity of the member. These interruptions provide tortuous paths for the passage of acoustic energy along the member. A plurality of masses are periodically spaced along the interior of the member and are each mechanically integral with opposite sides of the member at locations chosen to enable the member and masses to cooperate as a mechanical filter. By so doing, the structure made of the member and masses will have good acoustic delay and attenuation characteristics as well as good mechanical characteristics.
U.S. Pat. No. 5,043,952, titled “Monopole Transmitter For a Sonic Well Tool”, issued to David C. Hoyle, et al., relates to a monopole transmitter for a sonic tool which includes an axial tube, a piezoceramic cylinder surrounding the axial tube, an endcap disposed at each end of and firmly contacting the cylinder, and an apparatus for holding the endcaps firmly against the axial tube. The endcaps firmly contact the axial tube without simultaneously contacting an upper bulkhead. The apparatus may include spring washers disposed between the bulkhead and at least one endcap, or it may include a spring disposed between a nodal mount and each endcap. A nodal mounting tube may be disposed around the axial tube, a ring being disposed at each end of the nodal mounting tube, each ring being disposed outside of the cylinder for biasing the endcaps in tension against a ring thereby holding each endcap firmly in contact against the axial tube.
U.S. Pat. No. 5,510,582, titled “Acoustic Attenuator, Well Logging Apparatus and Method of Well Logging”, issued to James R. Birchak, et al., relates to a sonic well tool for performing acoustic investigations of subsurface geological formations penetrated by a borehole. The well tool generally includes a longitudinally extending body for positioning in the borehole. The tool also includes a transmitter supported by the body for transmitting acoustic energy and a receiver supported by the body for receiving acoustic energy. The tool includes an acoustic attenuation section positioned on the body between the transmitter and the receiver. This section includes one or more cavities defined by the body, inertial mass members positioned inside the cavities in a suitable manner to form a gap between the wall of the cavity and the inertial mass members, and an acoustical attenuation fluid in the gap. The method for attenuating sonic waves generally includes transmitting a sonic wave from the transmitter to the tool, passing the sonic wave through the acoustic attenuation section, and receiving attenuated wave at the receivers.
U.S. Pat. No. 5,036,945, titled “Sonic Well Tool Transmitter Receiver Array Including an Attenuation and Delay Apparatus”, issued to David C. Hoyle, et al., relates to a sonic well tool that includes a transmitter array having at least one monopole transmitter and at least one dipole transmitter and a receiver array for receiving sonic pressure wave signals from a surrounding borehole formation. A first attenuation and delay apparatus is positioned above the receiver array and a second attenuation and delay apparatus is positioned below the receiver array in the sonic well tool. The first attenuation and delay apparatus includes an attenuation member comprising a plurality of interleaved rubber and metal like washers for attenuating compressional and flexural waves propagating along a metal center support rod to the receiver array and an inner housing comprising a bellows section having a corrugated shape and a thin transverse dimension for delaying the propagation of compressional and flexural waves along the inner housing to the receiver array. The second attenuation and delay apparatus includes a plurality of mass loading rings surrounding the outer housing of the sonic well tool for attenuating the flexural waves propagating up the outer housing from a sonic,transmitter ad a further inner housing including a further bellows section having a corrugated shape and a thin transverse dimension for delaying the propagation of compressional and flexural waves up the tool, along the inner housing, to the receiver array. The sonic well tool also includes a differential volume compensator for changing the quantity of oil encapsulated in the sonic well tool in accordance with changes in oil volume and changes in borehole temperature and pressure. The receiver array includes a plurality of hydrophone sets, each hydrophone set including at least one pair and preferably two pair of hydrophones disposed in a cross section of the tool, one hydrophone of a pair being disposed opposite the other hydrophone of the pair in the cross section.
U.S. patent application Ser. No. 09/201,988, now U.S. Pat. No. 6,082,484 to Molz & Dubinsky, having the same assignee as the present invention discloses the use of a section of a drill collar that has a plurality of shaped cavities filled with oil. The passage of an acoustic wave sets up a resonance of the fluid in the shaped cavity. The frequency of resonance depends upon the shape and size of the cavity and the properties of the fluid in the cavity. In one embodiment of the invention, the cavities are spherical. Another embodiment of the invention uses cylindrical cavities with a piston restrained by a spring within the cavity. Changing the spring constant provides additional control over the frequencies that are attenuated. The Molz patent also discloses the use of segmented isolators in which the drill collar section is filled with layers of a composite material in which the layers have a different density. The thicknesses of the individual layers is selected to attenuate certain frequencies.